The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Many applications in research and industry benefit from high-resolution structural and spectral information obtained with scanning transmission electron microscopy (STEM) diffraction, high-angle annular dark-field STEM (HAADF-STEM), STEM electron energy loss spectrum (STEM-EELS), and STEM energy-dispersive x-ray spectroscopy (STEM-EDX). A typical operating sequence is to set up the instrument to acquire the signal of interest, place a finely focused electron probe on one spot on the specimen, acquire the signal (e.g. a diffraction pattern, a current reading from an HAADF detector, or a loss spectrum), and then move the probe and repeat the above operations. While automated systems allow this all to happen fairly quickly in a well-defined, regular 2D array, the information return is far from optimal. In the case of STEM diffraction and STEM-EELS, each acquisition may take a significant fraction of a second or even multiple seconds, so that the entire scan of the sample may take hours. Yet much of the information so obtained is in some sense redundant. Also, HAADF-STEM scans may be performed much more quickly than STEM-diffraction scans but return vastly less information; given a STEM-diffraction data set, one can reconstruct what any conventional STEM image (HAADF, conventional bright-field, annular bright-field, split-detector, etc.) would have produced while also using information from Bragg diffraction to identify crystal phases and orientations. Thus a technique that provides data return similar to STEM-diffraction with total exposure times comparable to HAADF-STEM would represent a substantial improvement relative to both techniques.
It is also well known, especially for STEM-EELS, that there are representations of the data for which the data set is “sparse.” What this means is that each spectrum or diffraction pattern can be represented to high precision as a combination of a relatively small number of principal or independent components, and that many of the spectra and diffraction patterns will look very similar to one another. From an information-theoretical perspective, this means much of the signal acquired in the scan is redundant and is not actually providing new, relevant, independent information about the sample. For HAADF-STEM, much faster scans are possible but only a small amount of information is retrieved from each electron; more specifically, either the electron hits the HAADF detector or it does not. This is because the very existence of HAADF represents a practical compromise. It is in essence a STEM diffraction system operating with a camera that is very fast but that only has a single pixel.
Past efforts to improve STEM data throughput have focused on improving the brightness, stability and aberrations in the probe-forming system as well as the signal-to-noise ratio in the detectors. Now that the signal-to-noise ratios are reaching the level of detecting single electrons, and the probe current densities are high enough that beam damage to the sample is very often the resolution limitation, these strategies are running out of room for improvement. Accordingly, new systems and methodologies are needed to make more efficient use of the acquisition time and the electron beam dose delivered to a sample when performing STEM diffraction or STEM-EELS.